Fix: Remove episodes from missing list on download/rescan

- Update _update_series_in_db to sync missing episodes bidirectionally
- Add delete_by_series_and_episode method to EpisodeService
- Remove downloaded episodes from DB after successful download
- Clear anime service cache when episodes are removed
- Fix tests to use 'message' instead of 'detail' in API responses
- Mock DB operations in rescan tests

tests/integration/test_download_flow.py

@@ -220,7 +220,8 @@ class TestDownloadFlowEndToEnd:
         
         assert response.status_code == 400
         data = response.json()
-        assert "detail" in data
+        # API returns 'message' for error responses
+        assert "message" in data
 
     async def test_validation_error_for_invalid_priority(self, authenticated_client):
         """Test validation error for invalid priority level."""